Magnetic float type liquid level gauges

ABSTRACT

Liquid level indicators include an upright cylindrical tube of non-magnetic material, a cylindrical, permanent first magnet element aligned with and movable coaxially within the tube, a float, and an annular, permanent second magnet element aligned with and movable coaxially outside the tube. The first and second magnet elements have magnetic fields in alignment with one another, with one of the first and second magnet elements being carried by the float. The first and second magnet elements are axially spaced relative to one another such that the one magnet element which is carried by the float supports by magnetic attraction the other magnet element which is not carried by the float.

This invention relates to a liquid level indicating gauge apparatus ofthe kind which includes, in combination, an upright cylindrical tube ofnon-magnetic material, a first permanent magnet which is movable axiallywithin and along the tube and a second permanent magnet which ismagnetically coupled to the first magnet and is movable along theoutside of the tube, and wherein one of said magnets is incorporated ina float that is adapted and arranged to float at the surface of aliquid, so that as the floating magnet rises and falls, the secondmagnet follows, rising and falling in sympathy, and thereby indicatesthe level of the surface of the liquid supporting the floating magnet.

In such arrangements as have been hitherto proposed, either the tube isimmersed in the liquid whose level is to be measured of DE-A-1 139 660,or U.S. Pat. No. 5,020,367, for example; or in a converse arrangement,of DE-A-1 034 877, for example, the tube will contain such liquid. Thusthe float may be in the form of a ring or collar surrounding the tubewithin which is located the second magnet; or in the conversearrangement, the floating magnet will be arranged within the gauge tubeand a second magnet will be arranged in a follower arranged externallyof the gauge tube.

Thus it is known to provide a gauge system where a float rises and fallsinside a sealed gauge tube of non-magnetic material, such as austeniticstainless steel, and which tube is connected, in sealed manner, to atank or receiver the liquid level of whose content is to be measured orindicated. Such a tank and gauge may for example contain inflammable ortoxic fluid, or it may be part of a pressurised system. The pressureresisting and integral construction of a gauge of this type renders itparticularly safe and suitable for use in marine, process engineeringand many other services.

Where magnetic forces are used to couple two magnets, one arrangedwithin, and the other arranged without the gauge tube, the magneticcoupling forces needed for one magnet reliably to follow the secondmagnet are very strong; and this is particularly the case with systemsdesigned to be secure and reliable, so that shock or vibration cannotreadily dislodge the follower magnet from its correct position.

And it is known to employ in such gauges, internal and external magnetswhich are magnetically coupled, either using forces of magneticattraction, or using forces of magnetic repulsion. However where, as inDE-A-23 42 735, strong magnetic forces are employed, there are generallypresent significant radial forces pulling the coupled magnets towards oraway from one another. This gives rises to severe instability of themagnetic array; with the inner magnet being urged laterally towards theside wall of the gauge tube, while equally strong forces pull the outermagnet towards the outer side of the tube. These radial forces give riseto friction which can greatly impair the accuracy of the level gaugeindication. Indeed, the prior art does not disclose a practical andcommercially usable system which provides for a magnetically stablearray such that one magnet reliably follows the movements of the other,with high frictional forces being avoided.

Accordingly, it is the main purpose of this invention to provide aliquid level gauge tube device of the general kind referred to, with thetube having inner and outer coupled magnets, but in which the magnetsare arranged in a magnetically stable array, with frictional forces setup between the float magnet and the tube wall on the one hand, andbetween the follower magnet and the tube wall on the other hand, beingminimised, and with the system being such as to provide for a reliable,rapid and sensitive response to liquid level change causing floatmovement.

A further preferred requirement of such a device is that the visualindication of level should not be restricted substantially to a front onview of the external follower system. And it is preferred that thedevice should be light, simple and economic and should also lend itselfto remote signalling; and preferably it should be capable of beingadapted to indicate high and low level alarm positions.

Broadly, this invention provides a liquid level indicator whichcomprises, in combination, an upright cylindrical tube of non-magneticmaterial, a first permanent magnet movable coaxially within the tube,and a second annular permanent magnet movable coaxially outside thetube, both magnets having magnetic polar axes which are in parallel withthe axis of the tube, and both magnets having like poles uppermost, andwith one said magnet being carried by a float resting at the surface ofthe liquid whose level is to be indicated, and the magnets being sospaced that the magnet carried by the float supports the other magnet bymagnetic repulsion at a level above that adopted by the floating magnet.

Preferably, a third permanent magnet is provided, which third magnet ismechanically linked either to said first permanent magnet or to saidsecond permanent magnet and is of similar form to that of the magnet towhich it is linked, and whose magnetic field is aligned in the samedirection as the first and second magnets.

The first magnet may be carried by the float and the float is slidablewithin the tube, and the second magnet is attached to a liquid levelindicator sleeve slidable along the outside of the tube.

According to another aspect of this invention there is provided a liquidlevel indicating gauge apparatus which includes, in combination, anupright cylindrical tube of non-magnetic material, a first permanentmagnet of cylindrical disc-like or ring like shape movable axiallywithin and along the tube and a second permanent magnet of annular shapesurrounding the tube and movable axially along the outside thereof; andwherein both said magnets are so magnetized and maintained in suchorientation relative to each other and to the tube as to have the samepolarities in the field directions with their magnetic poles beingaligned along the tube axis or in parallel therewith, and wherein one ofsaid magnets is incorporated in a float that it is adapted and arrangedto float at the surface of a liquid, while the other of said magnets issupported by magnetic repulsion at a level just above that adopted bythe floating magnet, with said magnetic repulsion having both axial andradial components. Due to these components, both axial and radial, ofthe forces of magnetic repulsion, it is possible to arrange the coupledmagnets, one supported by the other, in a magnetically stable array, andwith the radial force component providing what equates to a“self-centreing” effect; or an effect where the magnets, one inside andone outside the gauge tube, tend to find positions which aresubstantially coaxial with the tube axis.

Either the tube will be immersed in the liquid whose level is to bemeasured; or the tube will contain such liquid. The floating magnet maybe carried by a float which is in the form of a collar surrounding thetube; or the floating magnet will be mounted in a float arranged withinthe gauge tube. In either case the second, follower magnet will besupported by magnetic repulsion at a higher level and on the oppositeside of the tube wall from the floating magnet.

In the preferred arrangement, as so far described there have been twomagnets, one being float mounted and the other being arranged to followmovements of the first. However in an alternative arrangement, there arethree such permanent magnets arranged to be movable coaxially, back andforth, generally along said tube axis. All said magnets are magnetizedso as to have the same polarities in the field directions along and inparallel with the tube axis; and two of said magnets are arranged pairedalong said axis at a fixed distance from one another, while the thirdmagnet is maintained generally coaxial with the magnets of said pair,and by magnetic repulsion, at a level between them. In such an array,the third magnet is supported by magnetic repulsion by the lower of theother two magnets.

Where the follower magnet is arranged outside the gauge tube, itconveniently surrounds the gauge tube. The magnet may be of multi-partring construction and it may be housed in a multi-part indicator sleevewhich slides up and down along the gauge tube and is clearly visiblefrom all directions around the gauge unless masked by the body of thetank or receiver to which the gauge is connected. A clearly visible markor marks can be provided on the sleeve and these will be so positioned,calibrated and chosen that they will indicate the correct position ofthe liquid level relative to the follower ring. This relative positionmay vary for example, firstly in accordance with the field strengths ofthe magnetic assembly and secondly in accordance with the density of theliquid in the gauge and hence the depth of immersion of the float in theliquid. Accordingly, appropriate calibration will be made and selectionmade from a range of marks provided on the indicator sleeve, by means ofwhich the respective liquid levels can be indicated, as adjusted forgiven densities.

Inside the gauge there may be a cylindrical float which incorporates apair of circular annular or disc shaped magnets spaced apart by aspecified distance relative to their thickness. The levels of themagnets within the float may be adjustable. The disc magnets onceadjusted for level, are firmly held in the float and are so disposedthat their adjacent faces have opposite polarities.

The invention will be further understood by reference to theaccompanying drawings in which:

FIG. 1 is a diagram showing the magnetic force fields which exist whentwo magnets are located within a gauge tube and a further magnet islocated surrounding the gauge tube at a level between the two innermagnets.

FIGS. 2 and 3 are diagrams illustrating the potential energy whicharises in two contrasting magnet pole arrays for arrangement in a gaugetube.

FIG. 4 is one embodiment of a liquid level gauge according to theinvention, the assembly being shown in vertical cross-section.

FIG. 5 is an enlarged frontal view showing one embodiment of indicatorsleeve incorporating a follower magnet.

Reference will be made first to FIG. 4, where a gauge tube 10 has itsbore connected by means of top and bottom unions 11 and 12 with theinterior of a tank part of whose wall is marked 13. The tank, the tube10 and the unions 11 and 12 may form a sealed pressure resistantcontainer for the liquid which latter may be harmful to the extent thatit is a requirement that its escape must be prevented.

The tube 10 according to the invention is of non-magnetic material. Forexample it may be made of austenitic stainless steel in which case theliquid in its bore and the level of the surface thereof will beinvisible, the tube wall being opaque. Such liquid level in the gaugetube corresponds to the level of liquid in the tank and the object ofthe invention is to provide an indication of the level notwithstandingthat such level is invisible due to the opacity of the tube 10.

According to the illustrated example of the invention there is providedin the bore of the tube 10 a float 20 which is an elongate hollowcylindrical body of a plastics material. Mounted within the hollowinterior of the float 20 are a pair of magnets 21 and 22 held spacedapart on a rod 23 which depends, in this example, from the upper end ofthe float.

The float 20 is an easy sliding fit in the bore of the gauge tube 10,the float having at each of its ends, a ring of small projecting pips orprotrusions 25. These protrusions 25 make point contact when touchingthe interior wall of the gauge tube 10, helping to allow easy movementof the float 20 along the bore of the tube 10 as the level of the liquidrises and falls. The protrusions 25 are arranged in two spaced rings oneat or near each of the opposite ends of the external cylindrical surfaceof the float.

The magnets 21 and 22 are strongly magnetized permanent magnets whichmay be cylindrical discs or rings. They are magnetized so that theirpoles lie generally on the axis of the cylindrical float 20 and of thetube 10. Both magnets are magnetized to have the same polarity; forexample they may be polarized so as to have their North poles uppermost.The positions of the magnets 21 and 22 axially along the rod 23 may beadjustable. Such adjustment may be employed to calibrate the float toadjust for liquid density.

Mounted easily slidable along the outside of the gauge tube 10 there isan indicator sleeve, generally designated 30. The sleeve 30 is made likethe float 20 of non magnetic material and it may be of a plasticsmaterial such as glass reinforced resin. It may have on its inside smallprotrusions like the protrusions 25 on the outside of the float 20, andthese on the inside of the sleeve will make point contact with theoutside wall of the gauge tube, in order to reduce the area of contactwhile permitting easy sliding movements of the sleeve along the tube.

The sleeve 30 is of elongate cylindrical shape and as shown in FIG. 5 itis of multi-part construction comprising two half sleeves 31 and 32which are secured together by screws 33. Housed within a gallery formedby the half sleeves 31 and 32, is a strongly magnetized permanent magnet35 in the form of a ring which is itself formed by two C-shapedhalf-rings. The magnet 35 is magnetized axially and with the samepolarity as the magnets 21 and 22 mounted in the float. All threemagnets will have like poles facing in the same direction; and thus forexample, all three magnets 21, 22 and 35 may have their North polesfacing upwards in the arrangement shown in FIG. 4. The same arrangementof a gauge tube 10 with an array of three magnets, all having like polesfacing in the same direction, and the magnetic force field pattern whichoccurs as a result, is shown in FIG. 1.

In the system of FIG. 1, the three magnets employed are all permanentmagnets and they are all axially magnetised to have identical fielddirections which lie along the axis of the gauge tube 10 and they allhave their North poles facing upwards. It will be noted that thepolarities at the faces of the ring are unlike those of the adjacentfaces in the float magnets; and thus in FIG. 1, the top face of the ringmagnet 35 is a North pole which faces towards a South pole at theunderside of the top disc magnet 21, while a South pole at the bottomface of the ring magnet 35 faces towards the upward facing North pole ofthe bottom disc magnet 22 mounted in the float 20. When magnetic polesare directly opposed one to another, the governing rule is that likepoles repel while unlike poles attract. However, in the present case,the magnets 21 and 22 are inside the tube 10 and are of smaller diameterthan the magnet 35 which is located outside the tube 10; and thus thepoles are not directly opposed. Instead a consideration of the forcefield applicable, shows that the ring 35 is supported by the ring 22 byforces of magnetic repulsion. This may seem at first sight to be counterintuitive but it derives from the particular array adopted, and theinteraction of the magnetic force fields. Tests have in fact shown thatthe arrangement functions very well and that there is a region wherethere exists a low or minimum level of Potential Energy, that is anenergy well, which is located generally in the central plane between thetwo disc magnets 21 and 22. Axial displacement of the ring or followermagnet 35 from its central position, in either axial direction, meetsincreasing resistance so long as the dispositions of the float magnetsand follower ring magnet remain substantially coaxial.

The reason for the reliable functioning of this system will beunderstood if consideration is given to the the properties anddirections of the magnetic lines of force which surround all spacearound magnets. FIG. 1 is a diagrammatic representation of theconfiguration of the lines of force in the arrangement; and it alsoshows the energy well 41 where the axial Potential Energy is at aminimum, this being shown by plotting Potential Energy PE in the axialdirection along the gauge tube, as at the side of the drawing. Magneticlines of force may be taken to run from the North pole to the South poleand attraction forces make themselves felt along them.

It will be seen from FIG. 1 that the disc magnet 21 will be surroundedby toroidal lines of force leading from the North pole face to the Southpole face, and these lines of force will be closely bunched (i.e. ofhigher intensity) close to the disc, and less closely disposed withprogressively weaker forces in the regions further removed from thedisc. The ring magnet 35 also will have twin toroidal half spools oflines of force surrounding it. One half spool will lead from the Northpole face of the ring to its South pole face on the inside of the ring,and the other half spool will lead likewise, on the outside of the ring.

It is the case that forces of magnetic repulsion arise transverselybetween lines of force so long as these run parallel to each other. Asshown in FIG. 1, across a line at an angle of between 30° & 45° to thegauge tube axis, drawn from the lower edge of the top disc magnet 21 tothe centre of the upward pole face of the ring magnet 35, that thetorodial lines of force for both the disc magnet 21 and the ring magnet35 will run in the same parallel direction, and therefore that forces ofmagnetic repulsion will occur and will make themselves felt along thisline. As the disposition of these repulsive forces is circumferentiallysymmetric about the magnet axes, there will be a resultant repulsiveforce indicated by the arrows 37, tending to separate the ring magnet 35from the disc magnet 21 in an axial direction. On the opposing side ofthe ring magnet 35 there will be a similar opposing configuration offorces of magnetic repulsion, leading to a resultant repulsive forceindicated by the arrows 38. Due to the combined effects of theserepulsive forces, as depicted by the arrows 37 and 38, the ring magnet35 (disregarding its weight) will tend to adopt an axial position whichis substantially midway between the disc magnets 21 and 22; and anyaxial displacement of the ring magnet 35 from this mid position, ineither direction, will meet increasing resistance and it will always bepushed back to its middle position of minimum Potential Energy. Due toits weight the magnet 35 will in practice adopt a level which is lowerthan a mid position between the levels of the two inner magnets 21 and22.

In the arrangement of FIG. 1, it is the inner magnets 21 and 22 whichare float mounted while the outer ring magnet 35 is a follower magnet.However, the invention extends to the converse arrangement where themagnet or magnets disposed outside the gauge tube is/are float mounted,while the magnet or magnets disposed within the tube respond or followthe movements of the outer magnet or magnets.

There will also be magnetic attraction between the upward facing Northpole of the ring magnet 35 and the downward facing South pole of thedisc magnet 21, but because of the large axial distance between thesefaces, these forces will be weak and they will in any case becounteracted by opposing attraction on the other face of the ring magnetand the bottom disc. While it is true that as regards these forces ofattraction, the location of the ring magnet 35 at a middle positionbetween the discs 21 and 22 will not necessarily be a stable position ofminimum Potential Energy, the forces of magnetic attraction will be atlow intensity and they will not be able to disturb the stableequilibrium position resulting from the repulsion forces due to theinherent field of lines of force surrounding the disc magnets and thering magnet.

The axially stable position of equilibrium between the ring magnet 35and the disc magnets 21 and 22, as described, is also a position inwhich the ring magnet experiences no or only minimal radial forces whichmight cause friction between the outside indicator follower and thegauge tube on the one hand, and the float and inside wall of the gaugetube on the other hand. As has been explained, the position described isone of equilibrium between forces of magnetic repulsion, and arelatively small movement radially from the true coaxial position willgive rise to increased repulsion tending to return the ring to itscoaxial position. This arrangement is therefore particularly beneficialfor the purpose of magnetic gauge operation because the follower firmlyfollows the movement of the float up and down always in the middleposition between the disc magnets and yet there are no, or only verysmall, radial forces tending to produce friction which might render theoperation of the level indication sluggish. Practical tests show thatthe operation of the gauge with the magnetic coupling as described isboth reliable and very sensitive.

In particular, the arrangement of the magnets adopted in FIG. 1 and inFIG. 4, provides for forces of magnetic repulsion acting radially of thetube axis which tend to centralize the magnets both inside and outsidethe tube. This tendancy of “self centering” has the important effectthat if the internal magnet or magnets can be maintained at locationswhich coincide with the axis of the tube, then contact of all magnetswith the tube walls will be minimized.

Referring to the embodiments of FIG. 1 and 4, the description above hasbeen on the assumption that all the magnets are polarized with Northpoles facing upwards. However it will be apparent that the conversearrangement, where all the magnets are polarized to have South polesfacing upwards, will function equally well. When considering theappropriate polarization it is the disposition, interaction andfunctioning of the force fields which is important.

Further aspects of the nature of the magnetic coupling system asdescribed can also be understood if the effect of a single disc magnetis considered acting in conjunction with a ring magnet is considered inrelation to the arrangement shown diagrammatically in FIG. 2, where aring magnet is seen coaxial with and lying in the same axial plane as adisc magnet. If both magnets will have the same polarity in any givenaxial direction, there will be on the one hand intense radial repulsionforces between the ring and the disc tending to hold the ring in acoaxial position; and on the other hand the axial equilibrium will beunstable, because any small movement axially in any direction willincrease the distances between like poles. The position in fact will beone of Potential Energy maximum from which the ring will tend to tryescape in either axial direction if allowed to.

FIG. 2 represents the disposition of two magnets as described, and alsogives a schematic indication of the axial and radial distribution of thePotential Energy of the follower ring magnet. If the magnet which islocated inside the tube is mounted on a float at the surface of a liquidwhose level is to be monitored, then the outer ring magnet will be thefollower which rises and falls in sympathy with changes of liquid level,and such outer ring magnet will be supported, by magnetic repulsion, ata level which is above that of the inner magnet. The configuration ofFIG. 2 may employ a single disc magnet mounted inside the float, andwith a single ring or follower magnet mounted in the indicator body orsleeve which slides on the outside of the gauge tube. It is of courseessential that the follower magnet is so arranged that it always takes aposition which is above the float mounted magnet. The repulsion betweenthe two magnets will be balanced by the weight of the follower orindicator unit. When liquid level rises, the float mounted magnet willlift and drive the follower magnet upwards; while when liquid levelfalls, the follower magnet will slide down the gauge tube under theeffect of its own weight.

If in the converse array, it is the inner magnet which is the follower,with the outer magnet being float mounted at the surface of a liquid ,then, the inner magnet, which will move within the tube, will besupported by magnetic repulsion, at a level which is above that of theouter float mounted magnet located externally of the tube.

While this is a simpler version of gauge, it has a disadvantage ascompared with the double ring or disc float mounted magnet arrangementdescribed with reference to FIG. 1, in that the height relative to thefloat is not as precisely defined, and in that the presence of any dirtor grime may cause increased sliding friction on the gauge tube, so thatthe follower magnet may be unable closely to follow the rising andfailing movements of the float mounted magnet.

Returning now to the double disc float mounted magnet arrangement ofFIGS. 1 and 4, and imagining the ring magnet first as if it was lying onthe same plane as one of the discs magnets 21 or 22, it will beunderstood that by displacing the ring towards the other disc it willmove from a region Potential Energy maximum to a point where thegradient to the Potential Energy peak over the other disc makes itselffelt. There will in fact be defined between the discs an axial positionfor a Potential Energy minimum; and with disc magnets of equal strength,this will lie at a level between the two float mounted magnets. Inpractice, with a vertical disposition of the gauge, the follower bodywill come to rest slightly at a level below the middle plans between thediscs, with increased repulsion between the ring magnet and the lowerdisc acting to counterbalance the weight of the follower and the magnetmounted thereon. This departure from the middle position will, however,be stable and both on the rise and fall of the float the follower magnetwill positively be pushed towards its new position of equilibrium.

The spatial and dimensional relationships of the float mounted magnetsand the ring magnet is of some importance for the reliable functioningof the device, and by way of example, a practical execution may bequoted wherein the disc magnets mounted in the float are 12.5 mm indiameter and 3 mm thick and they are spaced 40 mm apart. The ring magnetin this example has 35 mm internal diameter and 41 mm external diameterand is also 3 mm thick.

Other dimensions and sizes can of course be employed, but it isimportant that the distance between the disc magnets should berelatively large both in relation to the thickness of the discs as wellas the thickness of the ring magnet, also a relatively large differencebetween the internal diameter of the ring magnet and the discs issignificant and amounts to nearly 3:1 in the example given.

The magnetic coupling arrangements described above will be furtherunderstood if the configuration depicted in FIG. 3 is considered. Herethe inner float magnet and the outer follower magnet are oppositelypolarized. In such a case the position of the ring magnet in the planeof the disc magnet will be one of a Potential Energy minimum in theaxial direction. Any displacement in either direction axially will giverise to attraction forces between the unlike poles and these will have acomponent in the axial direction tending to return the ring to itscentral position in the plane of the disc. However, in this case therewill also be an unstable equilibrium in the radial sense and any slightradial displacement from the axial position will give rise to strongradial forces tending to increase this displacement due to theattraction in the unlike poles which increases on the side of thesmaller radial clearance and decreases on the side of the larger radialclearance. Where the inner magnet is float mounted, as will usually bethe case, this arrangement will cause radial friction to occur betweenthe follower sleeve and the outside of the gauge tube on the one hand,and the float and the inside of the gauge tube on the other hand. Suchfriction effects will make the functioning of the level indicationsluggish and unreliable.

While in the magnetic coupling arrangement above described withreference to FIGS. 1 and 2, radial forces between float and follower aresmall or non existent, it is nevertheless advantageous to keep the floatin as central a position in the gauge as possible. The float itself willgenerally have some clearance between its outer diameter and the bore ofthe gauge to allow easy rise and fall, the float as envisaged in thisspecification is advantageously provided with the small projecting pipsor protrusions 25 around its circumference calculated to touch the wallof the gauge tube to maintain the float in the desired coaxialdisposition; also such protrusions will prevent contact of the floatwith the wall over an excessive area and will provide room for allowingadequate liquid flow past the float on bobbing movement on the liquidlevel.

It is particularly advantageous to arrange the projecting pips orprotrusions 25 in two rings in planes close to the top and bottom edgeof the cylindrical float 20. These will be so arranged that when itslews over, the float will be prevented from touching the bore of thegauge tube with either its top or bottom edge where such may be present,i.e. in the case where there are no hemispherical ends to the float, butwhere the float is constructed with a top and bottom welded lip. Thefriction caused by such a welded lip may be excessive and the provisionof the rings of pips near the top and bottom edges prevents thisfriction being set up. Any contact between the float and the tube willbe limited to a minimal area being that of the pips 25 which are theonly parts of the float which make contact with the gauge tube wall.

The float 20 will ride at the surface of the liquid whose level is to bemeasured, at a height which will depend upon the density of that liquid.Accordingly it is necessary to provide for calibration for this; and forcalibration to compensate or allow for the forces of magnetic repulsion,and to allow for the weight of the follower sleeve assembly 30 with themagnet 35 housed therein. Adjustment for purposes of such calibrationmay be provided in part by moving the magnets 21 and 22 up and downalong the rod 23 within the float 20 as previously described. Or and asshown in FIG. 5, the follower sleeve 30 may be marked with a number, forexample five, of different level positions 46 which may respectivelycorrespond to different levels adopted by the float 20, which in turnwill be dependent upon the density of the liquid which supports thefloat; and as shown at 47 a marker which may be coloured, will be placedat an appropriate selected level position 46, when calibration iscompleted, to give prominence to a marking at the appropriate level.

As a yet further feature according to this invention the magnetic gaugecan be surrounded by a housing of, say, transparent plastic materialwhich will protect the outer surface of the gauge tube from damage ordirt. The transparent housing is indicated at 45 in FIG. 4, and it canconveniently be made up of half round transparent plastic sheetingclamped together along opposing longitudinal edges by means of clips orclamping strip profiles of extruded rubber or plastic material. At thetop and bottom ends of the housing this will conveniently seal againstsuitably formed faces from the gauge connection bodies which may also beprovided with sealing means to prevent the ingress of dirt or oil.

A magnetic level gauge according to this invention may also be providedwith sensors which will sense the position of the magnetic ring in thefollower body along the gauge tube. Such sensors may be based on theHall Effect Principle which gives rise to an electronic signal by theclose proximity of the strong magnetic field. Such sensors may beconveniently used for high and low level alarm systems as are frequentlyrequired in gauging applications. In order to distinguish the directionin which an alarm position may be traversed, say from normal to full orfrom over full to normal, the Hall Effect Sensors can be arranged inclosely spaced pairs which can operate a logic sequence giving rise to asignal when, say, the lower sensor is passed first and the upper second(high level alarm), but does not give rise to a signal when the topsensor is triggered first and the lower sensor second (level fallingfrom over full position).

Sensors of the type described may be attached to a gauge either alongthe clamping edges of the two housing halves or they may be disposedalong one or more rods passing from the top gauge connection to thebottom gauge connection and where their height can be adjusted to therequired position. Similar sensors, but not necessarily in closelyspaced pairs, can be disposed at various discrete positions along thegauge to give intermediate signal indication between high and low levelalarm positions. The gauge design as a whole will give a robust andserviceable arrangement which is economic in its execution and can meeta wide range of service requirements.

Several inversions of the systems shown in the drawings can beenvisaged. As mentioned above, the external gauge tube fitted to a tankcan become a tube inside the tank passing through the liquid. In such acase, the ring magnets surrounding the tube can be incorporated in afloat which floats on a liquid level or on some interface between twoliquids. What was in the system described as a float inside the tubewill now become a follower bobbin which rises and falls inside the tubein response to the movement of the ring magnet in the outside float. Thebobbins inside the tube could then serve as an external indication ofthe position of the float. For shallow tanks this could be in the formof a protruding pointer which rises and falls against a scale on top ofa tank. In deeper tanks electric or electronic means can be envisaged toindicate float position such as the movement of a contact along apotentiometer wire.

There may also be a case, especially in the case of an internal gaugetube in a tank, to have two spaced out rings outside the tube suitablyhoused in the float and have a single magnetic disc inside the tubemoving up and down with the float and being used as the remote indicatormeans. This could be an advantage due to the lower weight of a singledisc magnet which would be counterbalanced only by the repulsion fromthe lower of the external ring magnets. The method of functioning ofthis system would be entirely analogous to that described for externalgauge tube.

Finally, the disc magnets inside the gauge tubes may have holes in them,say to carry an indicating pointer reaching to the top of the tank, orthe disc magnets may altogether be formed as ring magnets still axiallymagnetised. Such an arrangement would work largely in a similar manneras a disc magnet but it may be less compact and efficient than a disc.There may, however, be constructional features in the design which wouldmake use of rings instead of discs desirable.

Each magnet of the gauge may be linked to or incorporated in aquasi-cylindrical member constituted as a cylinder slidable along withinthe tube or as a sleeve slidable along outside the tube; and preferably,each such quasi-cylindrical member has an axial length which is at least1.5 times the diameter of the face of the tube against which it slides.This imposes a limit to the extent to which the responsive member can beupset from a true axial alignment and thus promotes a free easy slidingmovement and reduces any tendency the respective members may have to jamagainst the tube.

Preferably, in embodiments provided with a said third magnet which ismechanically linked to another said magnet, those two linked magnets areaxially spaced (centre-to-centre) by a distance which is at least equalto the external diameter of the tube.

What is claimed is:
 1. A liquid level indicator which comprises: anupright cylindrical tube of non-magnetic material, a cylindrical,permanent first magnet element aligned with, and movable coaxiallywithin, the tube, a float, and an annular, permanent second magnetelement aligned with, and movable coaxially outside, the tube, whereinsaid first and second magnet elements have magnetic fields in alignmentwith one another, and wherein one of said first and second magnetelements is carried by the float while another of said first and secondmagnet elements is not carried by the float, and wherein said first andsecond magnet elements are axially spaced relative to one another suchthat said one of said first and second magnet elements carried by thefloat supports by magnetic attraction said another of said first andsecond magnet elements not carried by the float.
 2. An indicatoraccording to claim 1, wherein one of the first and second magnetelements includes a pair of permanent magnets of similar form one toanother having magnetic fields aligned with one another and with anotherone of said first and second magnet elements.
 3. An indicator accordingto claim 2, wherein said first and second magnet elements are movableaxially back and forth generally along an axis of the tube and arepermanently magnetized to exhibit the same polarities in a fielddirection along and parallel with the tube axis, and wherein said pairof permanent magnets of said one of the first and second magnet elementsare separated from one another by a fixed distance, and wherein saidanother of said first and second magnet elements is maintained generallycoaxially with and, by magnetic repulsion, at a position between saidseparated pair of permanent magnets.
 4. An indicator according to claim1, further comprising a liquid level indicator sleeve slidable along anexterior of the tube, wherein the first magnet element is carried by thefloat and the float is slidable within the tube, and wherein the secondmagnet element is attached to the liquid level indicator sleeve. 5.Liquid level indicating gauge comprising: an upright cylindrical tube ofnon-magnetic material, a first magnet element having a cylindricalpermanent disc or ring magnet movable axially within and along the tube,and a second magnet element having an annular shaped permanent magnetsurrounding the tube and movable axially along the outside thereof,wherein both permanent magnets of said first and second magnet elementsare magnetized and maintained in such orientation relative to each otherand to the tube as to have the same polarities in field directions withtheir magnetic poles being aligned along an axis of the tube or inparallel therewith, and wherein the first magnet element is incorporatedin a float that is adapted and arranged to float at the surface of aliquid, while the second magnet element is supported by magneticrepulsion at a level above that adopted by the floating permanentmagnet, with said magnetic repulsion having both axial and radialcomponents.
 6. Apparatus according to claim 5, wherein one of the magnetelements includes two permanent magnets of similar form one to another,and whose magnetic fields are aligned in the same direction one toanother, as well as to the other magnet element.
 7. Apparatus accordingto claim 6, further comprising a liquid level indicator sleeve arrangedslidable along the tube externally thereof, wherein the one of themagnet elements which includes two permanent magnets is carried by thefloat and arranged movable coaxially along the tube, and the othermagnet element is carried by the liquid level indicator sleeveexternally thereof under the influence of the permanent magnet of themagnet element carried by the float inside the tube.
 8. Apparatusaccording to claim 7, wherein the tube and the liquid level indicatorsleeve are enclosed within a transparent housing.
 9. Apparatus accordingto claim 7 wherein the liquid level sleeve carries markings at aplurality of levels, calculated to correspond to flotation levels of thefloat at the surface of the liquid in which the float floats, saidflotation levels varying in dependence upon at least one of thedensities of the said liquid, and/or said markings being calculated tocompensate for the level of the float supported magnet relative to thelevel of the magnet installed in the indicator sleeve, and includingindicator means for affording prominence to a marking at a selected oneof said levels.
 10. Apparatus according to claim 7, wherein theindicator sleeve is split in an axial plane and is formed by twoseparable half sleeves which can be fixed together when the half sleeveshave been assembled, each half sleeve carrying a respective half of thepermanent magnet of the appurtenant magnet element.
 11. Apparatusaccording to claim 7, wherein the float has an elongate cylindrical wallopposed to the internal wall of the gauge tube, and wherein contactbetween the float and the gauge tube wall is minimized by providing onthe cylindrical wall of the float small pips or protrusions, there beingtwo spaced rings of such protrusions on the float wall, these being thesole parts of the float which make contact with the gauge tube wall. 12.Apparatus according to claim 7, in which the two permanent magnets ofthe said one magnet element are mechanically fixedly linked to oneanother, the two linked magnets being axially spaced center-to-center bya distance which is at least equal to an external diameter of the tube.